Gamma radiation is a high energy form of electromagnetic radiation which is produced by nuclear transitions of radioactive materials. The energy level of gamma radiation typically ranges between about 100 keV and about 3000 keV.
In the context of oil and gas applications, gamma radiation may be produced by “naturally occurring” sources in a subterranean formation, such as potassium (K), uranium (U) and thorium (Th), collectively referred to as KUT or background sources. The presence of naturally occurring sources of gamma radiation may be indicative of a particular type of geological formation. For example, shales often contain naturally occurring sources of gamma radiation. Since shales also often contain petroleum deposits, the identification of naturally occurring radiation in formations may assist in locating these petroleum deposits.
Alternatively, gamma radiation may be produced by radioactive materials which are introduced into a borehole or subterranean formation. The introduction of radioactive materials into a borehole or subterranean formation may facilitate a determination, by measuring the gamma radiation, of parameters relating to the borehole or subterranean formation, including the presence and/or location of fractures and the permeability of such fractures.
Gamma ray logging therefore involves the measurement of gamma radiation produced by either naturally occurring or introduced radioactive materials.
Gamma ray logging may be performed for the purpose of determining the total amount of gamma radiation which is present at one or more locations, zones or intervals of interest in a borehole, throughout all or a portion of the typical energy level range of gamma radiation. This type of gamma ray logging is often referred to as total gamma ray logging or total count gamma ray logging. Total count gamma ray logging may be used to determine the total amount of gamma radiation which is present at a particular location, zone or interval, but does not distinguish the gamma radiation on the basis of its various energy levels. Total count gamma ray logging is often performed during the drilling of boreholes by using “logging-while-drilling” systems.
Alternatively, gamma ray logging may be performed for the purpose of determining the amounts of gamma radiation which are produced at different energy levels throughout all or a portion of the typical energy level range of gamma radiation. This type of gamma ray logging is often referred to as spectral gamma ray logging, since a spectrum of gamma radiation as a function of energy level results from the logged data. Spectral gamma ray logging can be used to determine the total amount of gamma radiation which is present at a particular location, zone or interval, but may also be used to provide a breakdown of the amounts of gamma radiation that are present at different energy levels. This breakdown of the amounts of gamma radiation at different energy levels can provide useful information about the radioactive material or materials which are producing the gamma radiation. Spectral gamma ray logging is often used in conjunction with well evaluation or fracturing operations, where one or more radioactive materials may be injected into a well as tracers. Spectral gamma ray logging may also be used during the drilling of boreholes in order to obtain information about the amounts of naturally occurring radioactive materials which may be present in the borehole.
Every radioactive material exhibits a spectral signature, which is a gamma radiation spectrum of produced gamma radiation that is dependent upon the material and upon the environmental conditions. The spectral signatures of radioactive materials may be used to identify the radioactive materials that are the sources of gamma radiation which may be present at a particular, location, zone or interval. The amplitudes or magnitudes of the spectral signatures can be used to determine the amounts of the sources which are producing the gamma radiation.
Gamma radiation may be measured using a gamma ray detector. One common form of gamma ray detector is comprised of a scintillator and a photomultiplier. The scintillator receives gamma radiation and emits photons in response thereto. The number of photons which is emitted by the scintillator is proportional to the energy level of the gamma radiation received by the scintillator. The photomultiplier converts the photons into an electrical pulse which is proportional to the number of photons emitted by the scintillator and which is dependent upon the “high voltage” which is supplied to the photomultiplier.
The magnitude of the high voltage which is supplied to the photomultiplier establishes the “gain” of the gamma ray detector. The high voltage is typically adjustable in order to adjust the gain of the gamma ray detector.
Unfortunately, the gain of a gamma ray detector will typically fluctuate even if the high voltage remains fixed. Most significantly, changes in the temperature of the gamma ray detector will cause the gain to fluctuate.
The gain of the gamma ray detector defines the magnitude of the electrical pulse which is generated by the photomultiplier in response to the emission of photons by the scintillator. The magnitude of the electrical pulse which is generated by the photomultiplier is used to define the energy level of the gamma radiation which is received by the scintillator. Consequently, fluctuations in the gain of the gamma ray detector result in fluctuations in the apparent energy level or levels of the gamma radiation detected by the gamma ray detector.
The prior art describes systems and methods for compensating for the effects of change of temperature of a gamma ray detector. Many of these systems and methods provide for “gain stabilization”, which involves adjusting the gain of the gamma ray detector during data acquisition to compensate for the effects of temperature on the gamma ray detector. At least one system and method provides for a temperature correction of data acquired by the gamma ray detector.
European Patent Application No. 0 387 055 A2 (Gadeken et al) describes a spectral gamma ray logging method for obtaining relative distance indications of one or more radioactive tracers with respect to a wellbore. The method involves separating gamma radiation spectra into component parts attributable to each radioactive tracer and obtaining the relative distance indications for the radioactive tracers from the component parts of the gamma radiation spectra.
U.S. Pat. No. 4,220,851 (Whatley) describes a gain stabilization system which includes a light emitting diode positioned between the scintillator and the photomultiplier of a gamma ray detector, which light emitting diode emits stabilizing light pulses which are used to provide gain stabilization of the gamma ray detector.
U.S. Pat. No. 4,346,590 (Brown) describes an improved gain stabilization system of the type described in the Whatley patent which includes means for maintaining the intensity of the light pulses which are emitted by the light emitting diode regardless of temperature change.
U.S. Pat. No. 5,272,336 (Moake) describes a method for applying a temperature correction to radiation measurements made by a gamma ray detector, which method involves measuring the temperature when and where radiation measurements are taken and applying one or more temperature correction formulae to the measurements.
U.S. Pat. No. 5,461,230 (Winemiller) describes a method and apparatus for providing temperature compensation in gamma ray detectors which involves initially calibrating a gamma ray detector by placing a reference source of radiation in close proximity to the gamma ray detector and monitoring variations in the output signals from the gamma ray detector resulting from the reference source as a function of temperature. Once the initial calibration has been performed, a temperature compensation circuit uses a temperature sensor to vary the magnitude of a threshold voltage as a function of the operating temperature of the gamma ray detector in accordance with the initial calibration, wherein the threshold voltage corresponds to the gain of the gamma ray detector.
U.S. Pat. No. 6,051,830 (Moake) describes a method for calibrating a logging tool which includes providing a stabilization source which emits a stabilization signal having a known energy, receiving a total spectrum which includes the stabilization signal, discerning the stabilization signal in the total spectrum, and adjusting the gain of the logging tool on the basis of the discerned stabilization signal.
U.S. Pat. No. 6,554,065 (Fisher et al) describes a self-contained gamma radiation logging tool which includes a memory for storing data pertaining to detected nuclear energy.
U.S. Pat. No. 6,781,115 (Stoller et al) describes a system and method for detecting radiation in an area surrounding a wellbore, wherein the system provides azimuthally focused detector sensitivity.
U.S. Patent Application Publication No. US 2003/0138067 A1 describes apparatus and methods for measuring radiation in a borehole environment using a YAlO3:Ce (YAP) scintillation crystal.
U.S. Patent Application Publication No. US 2005/0199794 A1 describes a spectral gamma ray logging-while-drilling system and method for determining concentrations of naturally occurring radioactive materials in earth formations. The system and method provide automatic gain control for the gamma ray detector using two different methods of gain stabilization. A first method of gain stabilization involves developing a relationship between measured slope of a gamma ray spectrum in the Compton region as a function of the high voltage supplied to the gamma ray detector required to maintain a standard detector gain, and then using the slope to obtain a voltage adjustment as a first order gain correction which is required to maintain the standard detector gain. Once the voltage adjustment has been applied to the gamma ray detector for data acquisition, second and third order gain corrections are applied which involve adjusting the widths of energy channels and redistributing measured count rates so that ultimately, all identifiable peaks fall within standard energy channels. A second method of gain stabilization involves positioning a small radioactive source near the gamma ray detector so that the source generates a calibration peak in the measured gamma ray spectrum. The gain of the gamma ray detector is then adjusted throughout use of the system so that the calibration peak remains a constant energy level.
Finally, U.S. Pat. No. 5,608,214 (Baron et al) describes a gamma ray spectral tool and method which includes a self-adjusting gain stabilization feature and which is operable in a log/calibration mode and a latched mode. The gain stabilization feature is operative only when the tool is in the log/calibration mode. In the log/calibration mode, a voltage to a photomultiplier tube is dynamically adjusted to compensate for output shifts of the photomultiplier tube as indicated by 60 keV gamma rays from an americium source. In the latched mode, the voltage to the photomultiplier tube is held constant. When the output of the spectral tool indicates an overall quantity of gamma rays exceeding a predetermined value, the spectral tool is switched from the log/calibration mode to the latched mode.
There remains a need for a gamma ray logging system which facilitates calibration of detected gamma ray spectra and which does not require gain stabilization of the gamma ray detector during data acquisition. There also remains a need for a method of calibrating a detected gamma ray gamma radiation spectrum which is dependent only upon the detected gamma ray spectra.